This is a study of the maritime dimension of transport, to and from the military installations and civilian settlements of the coasts and rivers of the west of Britain, where water was used as the means of conveyance of military and commercial traffic during the Romano-British period. The study gathers together the various strands of evidence and, through systematic analysis, argues that the seas and rivers were a major factor in the Roman supply system and that, whilst the Roman system of roads has received much attention, insufficient consideration has been paid to the role of water transport.